1. Field of the Invention
The present invention relates generally to the fabrication of integrated circuits, and more particularly to a method for patterning a reflective surface in an integrated circuit.
2. Description of the Prior Art
Proper signal line patterning is important to the operation of an integrated circuit. Reflections off the surface of a metal layer during exposure of an overlying photoresist layer, however, can subsequently cause problems with the formation of the signal line.
Reflection problems are intensified on integrated circuits having hills and valleys on the surface. The sidewalls of the steps reflect the heat, light or radiation at various angles into the photoresist, resulting in poor image resolution. Poor image resolution is especially undesirable in complex integrated circuits, due to the importance of maintaining critical dimensions. Furthermore, those skilled in the art will recognize that notching, or a thinning of the signal line, typically occurs as a result of the reflections. Notching is especially a problem when the patterned signal line crosses a step. As known in the art, notching can lead to electromigration and other device failure mechanisms.
Techniques which have been used to solve reflection problems include the use of an anti-reflective coating on the surface of the metal layer, or on the bottom or the top of the photoresist layer. These techniques, however, are not completely successful in integrated circuits having a varied topography. As known in the art, the photoresist layer is thicker in the valleys on the surface of the integrated circuit. Therefore, it is necessary to overexpose the photoresist in order to allow the radiation to reach the photoresist located at the bottom of the valleys. This overexposure, however, can increase the amount of reflections from the metal layer.
Therefore, it would be desirable to provide a method for patterning reflective surfaces in integrated circuits which reduces the reflections into a photoresist layer. It is also desirable that such a method not significantly increase the complexity of the fabrication process.